Plasma processing apparatus

ABSTRACT

A plasma processing apparatus including a vacuum vessel, a lower electrode provided in the vacuum vessel to place a sample thereon, a matcher connected to the lower electrode, and a power supply for supplying power to the lower electrode via the matcher includes an electrostatic chuck electrode provided within the lower electrode to hold the sample, and a voltage measurement circuit provided within the lower electrode to measure a voltage at the electrostatic chuck electrode and output the measured voltage as a DC voltage.

BACKGROUND OF THE INVENTION

The present invention relates to a technique for manufacturingsemiconductors. In particular, the present invention relates to a plasmaprocessing apparatus suitable for conducting plasma processing onsemiconductor wafers by using plasma.

As the degree of integration of semiconductor elements becomes higher inrecent years, the circuit pattern goes on becoming finer. Accordingly,demanded working dimension precision is becoming stricter and stricter.Furthermore, the diameter of the wafer has become as large as 300 mmwith the object of reducing the manufacturing cost of semiconductorelements. However, it is demanded to make plasma uniform in a wide rangebetween the center of the wafer and the vicinity of the outer peripheryand make high-quality uniform working possible with the object ofincreasing the yield. In the product processing, it is typical to applya high-frequency bias in order to form a fine circuit pattern by usinganisotropy working. At this time, values of a high frequency voltage anda self bias voltage generated on the wafer are important parameters inworking. It becomes important to monitor them accurately.

In order to achieve such an object, it has heretofore been conducted todetect a high frequency voltage between a wafer and a matcher for a highfrequency power supply (see, for example, JP-A-2003-174015 andJP-A-2002-203835 corresponding to U.S. Pat. No. 6,771,481).

Apart from this, as to the influence of a high frequency transmissionpath upon the high frequency voltage, current and phase difference, itis known that a high frequency waveform in an output part of the highfrequency coupling circuit differs from that on the wafer andconsequently a wafer potential probe technique of directly measuring awafer potential to obtain information of the wafer potential iseffective (see, for example, JP-A-2001-338917).

Furthermore, in the conventional technique, in a parallel plate plasmageneration apparatus including an upper plate electrode of a metalmaterial and a lower wafer (functioning as an electrode), a highfrequency bias having the same frequency is applied to each of the upperelectrode and the lower electrode (wafer). A technique of monitoring thevoltages and phases of the upper electrode and the lower electrode inorder to control the high frequency voltage phase between those biasesis known (see, for example, JP-A-8-162292).

SUMMARY OF THE INVENTION

A phenomenon that poses a problem in the plasma processing apparatus isresonance caused by an inductance and a stray capacitance in a highfrequency power feeding system or a capacitance of an ion sheathgenerated on a front face of an electrode capacitance-coupled to plasma,such as the wafer. Resonance caused by the stray capacitance and theinductance of the power feeding system and resonance caused by thecapacitance of the ion sheath and the inductance of the power feedingsystem are independent of each other. In other words, the two resonancephenomena occur at the same time. This poses a problem that informationsuch as a voltage obtained from a measurement point indicates a valuethat is widely different from a state such as a voltage that is beingactually generated on the wafer or the electrode. A problem of theconventional technique is that these resonance phenomena are not takeninto consideration essentially.

Clearly, the technique described in JP-A-2003-174015 has a preconditionthat information obtained from a measurement point, such as a voltage,is the same as the information concerning the wafer or has the samequality as the information concerning the wafer. If this precondition isnot satisfied, the precision of the present technique is degradedremarkably.

In JP-A-2002-203835 corresponding to U.S. Pat. No. 6,771,481, attentionis paid to the fact that the precondition is not satisfied in thetypical plasma processing apparatus. In accordance with the presentinvention, it is possible to obtain information such as a load impedanceseen from the wafer, besides the voltage, current and phase on thewafer, from the information at the measurement point by preciselyspecifying an equivalent circuit between a measurement point for thevoltage or the like and the wafer. Even if the present technique isused, however, the influence of the resonance phenomenon in questioncannot be avoided. Because the inductance component and the straycapacitance causing the resonance are incorporated in an equivalentcircuit according to the present technique, but the capacitance of thepaired ion sheath is not incorporated into the equivalent circuit. Thisresonance phenomenon caused by plasma is a phenomenon that cannot bepredicted from the view point of the present technique.

In addition, it is very difficult, and substantially impossible, toincorporate the capacitance of the ion sheath into the equivalentcircuit and evaluate it accurately. Because the capacitance of thesheath depends upon plasma characteristics (an electron density, anelectron temperature and a gas density and distribution of them on thewafer), which in turn depend upon a large number of parameters such asthe gas pressure and gas component and high frequency power for plasmageneration, and high frequency power for bias applied to the wafer, andconsequently the capacitance value of the sheath cannot be calculatedaccurately. As a matter of course, there is a theory for calculating thecapacitance. However, it is not possible to know accurate values ofnumerical values to be substituted into the theory. In other words,precision assurance cannot be conducted.

Furthermore, the capacitance of the ion sheath is a major element thatdetermines a value of the load impedance seen from the wafer. A highfrequency voltage generated on the wafer depends upon a combination of acircuit ranging from a matching circuit to the wafer and the loadimpedance. However, the capacitance of the ion sheath has a propertythat it depends upon the high frequency voltage generated on the wafer.In other words, the capacitance and the wafer voltage depend on eachother and they are related by a nonlinear relation. As for determinationof the capacitance and the wafer voltage, therefore, it cannot be solvedby using ordinary equivalent circuit simulation. They cannot bedetermined without executing a recursive calculation using a numericalcomputation method. It is very difficult to conduct the presentcalculation in real time from the viewpoint of both aligning numericalvalues of basic data for calculation start and the calculation time.

A conclusion obtained from the foregoing description is that theresonance phenomenon in question cannot be solved by using the techniqueof using the equivalent circuit. As a result, calculation cannot beconducted even if the equivalent circuit is used, or the precisionassurance cannot be conducted.

As compared with the technique disclosed in JP-A-2003-174015 orJP-A-2002-203835 corresponding to U.S. Pat. No. 6,771,481, the techniquedisclosed in JP-A-2001-338917 is a technique of directly measuring thewafer potential and the resonance phenomenon in question can be avoidedin principle. However, the present technique has a problem ofreliability, and it is difficult to put the present technique topractical use. According to the present technique, an oxide film or anitride film located on the back of the wafer is broken through by ahard needle of WC (tungsten carbide), and direct measurement of thewafer voltage is implemented. A problem is that it cannot be ensured tobreak through the film on the back of the wafer certainly and implementstable measurement in a semiconductor manufacturing apparatus forprocessing five hundred thousand to one million wafers one afteranother. It is very difficult to design such a structure.

As regards the phase as well, it is well known that there is a largechange between phases before and after the resonance point and the phaseis inverted in an extreme case. Also in the technique of conducting thephase control as described in JP-A-8-162292, the control performance iscrippled by the resonance in question. The resonance in question is aphenomenon that the inductance of the high frequency transmission pathand the capacitance of the ion sheath cause resonance. The resonance inquestion is a phenomenon that occurs not only when a high frequency biasis applied to the wafer but also when a high frequency bias is appliedto the electrode opposed to the wafer as described in JP-A-8-162292. InJP-A-8-162292 as well, the resonance in question is not taken intoconsideration as regards the phase measurement point, and it isappreciated that the resonance phenomenon in question constitutes aserious hindrance in the same way as JP-A-2003-174015, JP-A-2002-203835corresponding to U.S. Pat. No. 6,771,481, and JP-A-2001-338917.

Hereafter, a resonance phenomenon found by the present inventors will bedescribed in detail. Here, an electrode mounting the wafer is taken asan example. However, these two resonance problems occur entirely in thesame way, as regards any electrode capacitance-coupled to plasma. First,it will now be described that a resonance phenomenon is seen even ifplasma is not present, by representing the electrode structure by theuse of an equivalent circuit and taking measurement of a voltage (here,a peak-to-peak voltage Vpp) as an example. This is first resonance,i.e., resonance caused by a stray capacitance and an inductance of thehigh frequency transmission system. Subsequently, a resonance phenomenonin the case where there is plasma will be described. This is secondresonance, i.e., resonance caused by the capacitance of the ion sheathand the inductance of the high frequency transmission system. Theentirely same conclusion is obtained as regards the phase measurement aswell.

The first resonance, i.e., resonance caused by the stray capacitance andthe inductance of the high frequency transmission system will now bedescribed. FIG. 1 schematically shows a block diagram of componentsranging from a wafer bias RF power supply to an electrode. Beginningwith an output of the wafer bias RF power supply, a matching circuit, aVpp detector, a power feeding cable, and an electrode are included inthe cited order. The components ranging from the RF power supply to thepower feeding cable are in the air, and the electrode for mounting thewafer is in the vacuum. A circuit shown in FIG. 2 is obtained byreplacing each of the blocks shown in FIG. 1 with an equivalent circuit.The power feeding cable is an ordinary coaxial line, and it includes aninductance (L1+L2) of a central conductor and a stray capacitance (C1).The electrode is divided into a high frequency transmission part (havingan equivalent circuit that is the same as the coaxial structure has) anda spray deposit (C3+R1) for electrostatic chucking on a wafer. A highvoltage probe (8 pF and 10 MΩ) for voltage measurement is connected tothe wafer. Since the impedance is very high and it can be neglected,however, it is not written in the equivalent circuit. The equivalentcircuit shown in FIG. 2 is a typical one. As for the actual electrode, alarge number of contrivances, such as a focus ring, have been executedand in addition, stray capacitances represented by Cs1 and Cs2 areincluded. Therefore, the equivalent circuit becomes more complicatedthan that shown in FIG. 2.

A result obtained by measuring frequency characteristics with theconfiguration shown in FIG. 1 by using the actual electrode is shown inFIG. 3. Its abscissa indicates a frequency applied as a bias, and itsordinate indicates a ratio between voltages in positions V1 and V2 shownin FIG. 2. It is appreciated that some resonance points appear atfrequencies of 4 MHz or above. Therefore, the inductance and capacitanceof the electrode are measured to generate an equivalent circuit, andsimulation is conducted. This result is shown in FIG. 4. It is foundthat the measured resonance phenomenon can be reproduced. This can beunderstood by means of the typically known resonant frequencyrepresented by the following expression (1).

$\begin{matrix}{{fo} = \frac{1}{2\pi \sqrt{L\; C}}} & (1)\end{matrix}$

In the equivalent circuit shown in FIG. 2, a total inductance Lt of thetransmission line is approximately 1.7 μH and a total stray capacitanceCt of the transmission line and the electrode is approximately 908 pF.Substituting them into the Expression 1 yields 4.1 MHz, and the resultof the measurement can be explained well. Although the resonancephenomenon itself can be reproduced by the simulation, the voltage ratiocannot be reproduced. This is because it is scarcely possible to replaceelectrical characteristics of the actual structure by such an accurateequivalent circuit that the measurement precision can be assured.

If resonance occurs at 4 MHz as heretofore described, then thereliability of voltage measurement conducted when using a frequencylower than the resonant frequency but higher than at least 2 MHz in thiscase, although it also depends on the resonance bandwidth (Q value). Itis important that the inductance Lt and the stray capacitance Ct arerespectively 1.7 μH and 908 pF, and consequently they are not extremelylarge values. They are the inductance and stray capacitance generatedeasily by connecting a high frequency transmission path having a lengthof several meters to the electrode. According to the experience of thepresent inventors, it is necessary to take this resonance phenomenoninto consideration when using a bias having a frequency of at least 1MHz, although it depends upon the design technique and the apparatusconfiguration.

The second resonance, i.e., the resonance caused by the capacitance ofthe ion sheath and the inductance of the high frequency transmissionsystem will now be described. If there is plasma, the wafer iscapacitance-coupled to the plasma. Therefore, it becomes necessary totake a new capacitance generated by the plasma into consideration. Inaddition, when there is plasma, it is conceivable that there is a casewhere the resonant frequency further falls as compared with the caseshown in FIG. 3 or 4. In this new capacitance, the capacitance of theion sheath formed on the front face of the wafer becomes dominant. Athickness d_(sh) of this ion sheath is theoretically given by thefollowing expression (2).

$\begin{matrix}{d_{sh} = {1.36\frac{\sqrt{2}}{3}{\lambda_{db}\left( \frac{2{eV}_{sh}}{k_{B}T_{a}} \right)}^{0.75}}} & (2)\end{matrix}$

Here, λ_(db) is the Debye length, e is the elementary charge, k_(B) isthe Boltzmann's constant, and T_(e) is the electron temperature. Anaverage voltage V_(sh) of the sheath can be defined by the followingexpression (3).

$\begin{matrix}{V_{sh} = {\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{V_{S}(\tau)} - {V_{B}(\tau)}} \right){\tau}}}}} & (3)\end{matrix}$

Here, τ is an angular frequency of the bias, V_(s)(τ) is a plasma spacepotential, and V_(B)(τ) is a bias potential.

The final capacitance of the ion sheath is represented by the followingexpression (4) using the thickness d_(sh) of the ion sheath.

$\begin{matrix}{C_{sh} = \frac{ɛ_{0}S_{W}}{d_{sh}}} & (4)\end{matrix}$

Here, ε₀ is the dielectric constant of the vacuum, and S_(W) is an areaof the wafer.

Since the area of the wafer is constant in the Expression 4, it isappreciated that the capacitance of the ion sheath is in inverseproportion to the thickness of the ion sheath. In other words, acondition under which the thickness of the ion sheath becomes thin isequivalent to a condition under which the resonant frequency becomeslow. The Debye length is the basic length of the electric fieldshielding capability, and it becomes short in inverse proportion to thedensity of the plasma. In the plasma, the electron temperature changesonly by several tens percents at most. Neglecting the electrontemperature change accordingly, it is appreciated from the Expression 2that a condition under which the thickness of the ion sheath becomesthin is satisfied when the plasma density is high and the bias voltageis low. A conclusion obtained from this is that the resonant frequencyin question is not constant but it changes depending upon the plasmageneration condition and wafer working condition even in the sameapparatus or if the apparatus is different.

Typically, in plasma used for working on semiconductor products, theelectron temperature is approximately 3 eV and the plasma density is inthe range of 10¹⁰ to 10¹² cm⁻³. Furthermore, the bias voltage is in therange of 100 to 4,000 Vpp. The capacitance of the ion sheath obtainedfrom this is in the range of approximately 200 to 8,000 pF. By usingthese values, the resonance is simulated. A schematic equivalent circuitis shown in FIG. 5. In the equivalent circuit shown in FIG. 5, a plasmaload is added to the equivalent circuit shown in FIG. 2. Letting C5=2000pF and R3=160Ω (corresponding to the wafer of 300 mm) as a typicalplasma circuit, a result shown in FIG. 6 is obtained. It is appreciatedfrom the result shown in FIG. 6 that the resonant frequency falls to 3MHz. As appreciated from FIG. 5, there is C3, which is a capacitance ofthe electrode spray deposit, in series with C5. A composite capacitanceof C3 and C5 causes resonance with inductances (L1 to L4) on thetransmission line. Supposing that C3=7500 pF (corresponding to the waferof 300 mm), the composite capacitance becomes equal to 1579 pF.Substituting this value and 1.7 μH which is the inductance Lt of thetransmission line into the Expression 1, a value of 3.1 MHz is obtainedand the simulation result is explained well. This indicates that theresonant frequency at the time when there is plasma depends on thecomposite capacitance of the capacitance of the ion sheath and thecapacitance of the electrode spray deposit and the inductance of thetransmission line. Since the capacitance of the electrode spray depositassumes a value unique to the apparatus, it can be concluded that theresonance phenomenon is generated by the inductance of the transmissionline and the capacitance of the ion sheath.

This is verified by actually using the apparatus. FIG. 7 shows frequencycharacteristics obtained when the wafer bias power supply is output soas to make Vpp on the electrode equal to a constant voltage of 20 V. Aspredicted from the theory, the resonant frequency becomes extremely low.In this case, the resonant frequency becomes 2 MHz or below. Supposingthat the inductance Lt of the transmission line is 1.7 μH incalculation, the composite capacitance is estimated to be approximately4,300 pF. Since Vpp is extremely low in this case, the capacitance ofthe sheath amounts to approximately 10,000 pF. It is appreciated thatthe resonant frequency falls remarkably when the bias voltage is low, aspredicted in accordance with the theory described heretofore.

Conclusions and problems obtained heretofore are put together. First,there are two resonance phenomena in question. The first resonancephenomenon is generated by the inductance and the stray capacitance ofthe high frequency transmission line. The second resonance phenomenon isgenerated by the inductance of the high frequency transmission line andthe capacitance of the ion sheath. On the basis of this principle, it isimpossible that the resonance phenomenon itself disappears. The resonantfrequency based on the capacitance of the ion sheath has strongdependence on the bias voltage and the plasma density. The resonantfrequency based on the capacitance of the ion sheath changes greatlydepending upon the wafer processing condition.

As a matter of course, it is more advantageous to lower theseinductances and capacitances because the resonant frequency is raised inaccordance with the (Expression 1). When the frequency of the highfrequency used as the bias is in the vicinity of the resonant frequency,the voltage value measured at the measurement point becomes widelydifferent from the voltage that is actually generated on the wafer.Furthermore, the ratio between the voltage at the measurement point andthe voltage on the wafer changes according to the wafer processingcondition, and the ratio does not assume a constant value. It issubstantially impossible to quantitatively calculate the voltagegenerated on the wafer, by using the equivalent circuit. As regards thephase and the current measurement as well, the conclusion is the same.

Dimensions of wafers in the semiconductor processing apparatuses, liquidcrystal substrates, and so on have been extended from the past to thepresent time. This aims at reducing the manufacturing cost. Thistendency can be expected to continue hereafter as well, although itdepends on the development of the technique as well. An increase indimensions, i.e., in area of the substrate such as the wafer lowers theresonant frequency, because it increases the capacitance of the sheathas represented by the Expression 4. Therefore, the technique provided bythe present invention becomes a technique indispensable to applying ahigh frequency in future semiconductor manufacturing.

An object of the present invention is to provide a technique by whichvoltage and phase measurement can be easily set to arbitrary goalprecision even under presence of the resonance phenomenon.

In order to achieve the object, in accordance with a first aspect of thepresent invention, a plasma processing apparatus including a vacuumvessel, a lower electrode provided in the vacuum vessel to place asample thereon, a matcher connected to the lower electrode, and a powersupply for supplying power to the lower electrode via the matcherincludes an electrostatic chuck electrode provided within the lowerelectrode to hold the sample, and a voltage measurement circuit providedwithin the lower electrode to measure a voltage at the electrostaticchuck electrode and output the measured voltage as a DC voltage.

In accordance with a second aspect of the present invention, a plasmaprocessing apparatus including a vacuum vessel, a lower electrodeprovided in the vacuum vessel so as to incorporate an electrostaticchuck electrode for holding a sample, a matcher connected to the lowerelectrode, and a power supply for supplying power to the lower electrodevia the matcher includes a voltage measurement circuit provided underatmospheric pressure to measure a voltage at the electrostatic chuckelectrode and output the measured voltage as a DC voltage, and a coaxialline for connecting the electrostatic chuck electrode to the voltagemeasurement circuit.

According to the present invention, it is possible to implement adetection circuit that is not susceptible to the influence of resonanceeven if the resonance is present. As a result, the high frequencyvoltage and phase can be detected accurately. Furthermore, it becomespossible to run the operation of the plasma processing apparatus stablyin an optimum state.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of components included in the range of a waferbias RF power supply to an electrode;

FIG. 2 is an equivalent circuit diagram for the block diagram shown inFIG. 1;

FIG. 3 is a diagram showing frequency characteristics in theconfiguration shown in FIG. 1;

FIG. 4 shows a result of simulation conducted by using the equivalentcircuit shown in FIG. 2;

FIG. 5 is an equivalent circuit diagram for a range from the wafer biasRF power supply to plasma;

FIG. 6 shows a result of simulation conducted by using the equivalentcircuit shown in FIG. 5;

FIG. 7 is a frequency characteristic diagram obtained when Vpp on theelectrode is set equal to a constant voltage of 20 V;

FIG. 8 is an equivalent circuit diagram obtained when a Vpp detector isincorporated into the electrode;

FIG. 9 is a schematic diagram showing a first embodiment of a plasmaetching apparatus;

FIG. 10 is a schematic diagram showing a second embodiment of a plasmaetching apparatus;

FIG. 11 is an equivalent circuit diagram for the configuration shown inFIG. 10;

FIGS. 12A-12C show results of simulation conducted by using theequivalent circuit shown in FIG. 11;

FIG. 13 is an equivalent circuit diagram obtained when a phase detectoris incorporated into the electrode;

FIG. 14 is an equivalent circuit diagram obtained when the phasedetector is disposed outside the electrode;

FIGS. 15A and 15B show results of phase difference simulation conductedusing the equivalent circuit shown in FIG. 14; and

FIG. 16 is a schematic diagram showing a plasma etching apparatusaccording to a third embodiment.

DETAILED DESCRIPTION OF THE INVENTION

As described above, the resonances do not disappear, and correctionusing a calculation or calibration cannot be conducted. Therefore, it isappreciated that it is important in attaining the object to configurethe apparatus so as to cause the voltage and phase information at themeasurement point to be equivalent to or have the same quality as thevoltage and phase information at an electrode of the measurement subject(an electrode capacitance-coupled to plasma on the wafer or the like).Specifically, it is important to form a configuration having a detectioncircuit that is not susceptible to the influence of resonance even ifthe resonances are present.

Such a configuration can be achieved by incorporating the Vpp detectorincorporated in the matcher shown in FIGS. 1 and 2 into the electrode.This configuration is shown in FIG. 8. According to this configuration,the Vpp detector becomes unsusceptible to the influence of L1 to L4causing the resonance and it becomes possible to convert a voltagegenerated directly at the electrode to a DC voltage and output the DCvoltage.

Hereafter, a first embodiment obtained by making the structure shown inFIG. 8 concrete will be described.

FIG. 9 is a longitudinal section diagram of an etching chamber used inthe present invention. In the present embodiment, an example of a VHFplasma etching apparatus for forming plasma by utilizing a VHF (VeryHigh Frequency) and a magnetic field is shown. An upper opening partincluding a cylindrical processing vessel 104, a platelike antennaelectrode 103 formed of a conductor such as silicon, and a dielectricwindow 102 formed of quartz and sapphire capable of transmittingelectromagnetic waves is placed on a vacuum vessel 101 via a vacuum sealmaterial 127, such as an O-ring, so as to be hermetically sealed. Aprocessing chamber 105 is formed inside. A magnetic field generatingcoil 114 is provided on an outer periphery part of the processingchamber 104 so as to surround the processing chamber. The antennaelectrode 103 has a perforated structure for letting an etching gasflow. A flon gas such as CF₄, C₄F₆, C₄F₈, C₅F₈, CHF₃ or CH₂F2, an inertgas such as Ar or N₂, or O₂ or a gas containing an oxide such as CO iscontrolled by a flow rate adjuster (not illustrated) including an MFC(mass flow controller) provided in a gas supplier 107, and led into theprocessing chamber 105 via the gas supplier 107. Furthermore, a vacuumexhauster 106 is connected to the vacuum vessel 101. The inside of theprocessing chamber 105 is kept at a predetermined pressure by a vacuumexhauster (not illustrated) including an MP (turbo-molecular pump)provided in the vacuum exhauster 106 and a pressure governor (notillustrated) including an APC.

A coaxial line 111 is provided over the antenna electrode 103. A highfrequency power supply for plasma generation (first high frequency powersupply) 108 (having, for example, a frequency of 200 MHz) is connectedto the antenna electrode 103 via the coaxial line 111, a coaxialwaveguide 125 and a matcher 109. A substrate electrode 115 on which awafer 116 can be disposed is provided in a lower part in the vacuumvessel 101. In the same way as the antenna electrode 103, a coaxial line151 is provided under the substrate electrode 115. A wafer bias powersupply (second high frequency power supply) 119 (having, for example, afrequency of 4 MHz) is connected to the substrate electrode 115 via thecoaxial line 151, a coaxial waveguide 152, a power feeding cable 153,and a matcher 118. The coaxial line 151 and the coaxial waveguide 152are, for example, the high frequency transmission part in the electrodeshown in FIG. 2, and they are in the vacuum. The power feeding cable 153is on the atmospheric pressure side. An electrostatic chuck electrode124 having an electrostatic chuck function for adsorbing the wafer 116electrostatically is buried in the substrate electrode 115. Anelectrostatic chuck power supply 123 is connected to the electrostaticchuck electrode 124 via a filter 122. The filter 122 passes through DCpower from the electrostatic chuck power supply 123, and effectivelycuts off power from the plasma generation high frequency power supply108 and the wafer bias power supply 119.

In the present configuration, a wafer voltage measurement circuit 154 isincorporated right under the electrostatic chuck electrode 124 in thevacuum. The influence of the resonance is eliminated by thus attachingthe measurement circuit directly to a place where the voltage to bemeasured is generated, converting the measured voltage to a DC voltageon the spot, and taking out a resultant signal to the outside of thevacuum. A composite impedance of C6 and C7 in the voltage measurementcircuit 154 shown in FIG. 8 must be sufficiently high. To which degreethe composite impedances must be high will be described with referenceto a second embodiment. However, this method has several problems. Theproblems are: (1) electric parts (such as resistors, capacitors, coilsand diodes) in use are premised on use in the atmosphere, and theperformance is not assured for use in the vacuum; (2) since heatgeneration from the electric parts is inevitable and little heat isradiated in the vacuum, continuous use is impossible; the possibilitythat part degradation will be caused by a corrosive gas is high; (4)when film deposition occurs, the possibility that circuit operation willbe affected is high; (5) the possibility that the circuit will bedamaged by turnaround of the high frequency for plasma generation ishigh; and (6) the possibility that the circuit will be damaged or thecircuit operation will be affected by plasma generated around thecircuit because of turnaround of the high frequency for plasmageneration is high. Each of these problems is not insoluble. Forexample, the problems can be solved by burying the whole of the voltagemeasurement circuit 154 into resin, housing the whole of the voltagemeasurement circuit 154 into a hermetically sealed structure to protectthe voltage measurement circuit 154 from the corrosive gas, and housingthe whole of the voltage measurement circuit 154 into a hermeticallysealed vessel that can be shielded electromagnetically.

A second embodiment in which the problems of the first embodiment aresolved more thoroughly is shown in FIG. 10.

In the present configuration, the voltage measurement point is theelectrostatic chuck electrode 124 in the same way as FIG. 9. Thisvoltage is taken out to the outside of the vacuum by using a coaxialcable 157. The voltage taken out to the outside of the vacuum isconverted to a DC voltage signal by using the voltage measurementcircuit 154. This configuration has a merit that the demerit of theconfiguration shown in FIG. 9 is eliminated because the voltagemeasurement circuit 154 can be disposed on the atmosphere side. Asregards the voltage measurement, the above-described resonancephenomenon loses no relation because it suffices that the voltage at theelectrostatic chuck electrode 124 is equal to the voltage at the voltagemeasurement circuit 154.

A special contrivance becomes necessary in the coaxial cable 157 and thevoltage measurement circuit 154 in order to make the voltage at theelectrostatic chuck electrode 124 equal to the voltage at the voltagemeasurement circuit 154.

An equivalent circuit for the apparatus shown in FIG. 10 is shown inFIG. 11. The equivalent circuit shown in FIG. 11 differs from theequivalent circuit shown in FIG. 8 in that a coaxial cable is insertedbetween the electrode and the voltage measurement circuit. Theabove-described special contrivance is to make a composite impedance Zsof the coaxial cable and the voltage measurement circuit sufficientlyhigher than a load impedance Zp inclusive of the plasma. If Zs is small,then a voltage drop is caused by Zs and large reactive current flows,resulting in a heavy burden on the transmission system. If an RF powersupply shown in FIG. 11 is controlled to output constant power, thensuch a demerit is not eliminated completely, but it can be suppressed toa negligible level in an allowable range.

The relation between Zp and Zs will now be described in detail. Whenseen from the RF power supply shown in FIG. 11, Zp and Zs are connectedin parallel as a load circuit. Therefore, a load impedance Z at the timewhen Zs is not coupled is Z=Zp, whereas a load impedance Z′ at the timewhen Zs is coupled becomes Z′=Zp·Zs/(Zp+Zs). On the other hand, when theRF power supply is used in power control, V1 which is the voltage to bemeasured is determined by V1=(WZ)̂0.5 where W is RF power. As a result,the ratio between a voltage V1′ at the time when Zs is coupled and avoltage V1 at the time when Zs is not coupled is represented byV1′/V1=(Zs/(Zp+Zs))̂0.5. Letting V1′/V1=α, α represents precision of themeasured voltage value in the state in which the voltage measurementcircuit is coupled. Thereafter, a is a value in the range of 0 to 1.From the foregoing description, the relation between Zp and Zs isrepresented by the following expression (5) using α.

$\begin{matrix}{{Zs} = {\frac{\alpha^{2}}{1 - \alpha^{2}}{Zp}}} & (5)\end{matrix}$

If, for example, the voltage detection precision is made at least 95%,then it is appreciated from this expression that Zs must have animpedance that is at least 9.3 times as large as Zp. It is also possibleto replace C6 and C7 in the voltage measurement circuit by resistors.Unless resistances of the resistors are sufficiently high (for example,at least 10 MΩ), however, power loss is caused in the resistors.Accordingly, care should be taken.

This will now be described by using concrete numerical values. Acomposite impedance Zp obtained by seeing the plasma side from the placeof V1 is calculated by using C5=2000 pF, R3=160Ω and other constants. Asa result, |Zp|=approximately 15Ω is obtained.

A composite impedance Zs obtained by seeing the voltage measurementcircuit side from the place of V1 will now be found. Supposing a coaxialcable corresponding to 3D2V from the viewpoint of the withstand voltage,inductance and capacitance per unit length become 0.27 μH/m and 103pF/m, respectively. These correspond to L5, L6 and C9 shown in FIG. 11.Supposing that composite capacitance of C6 and C7 is 8 pF and thecoaxial cable has a length of 1 m, it follows that Zs=−355i Ω, where iis an imaginary number. It follows that |Zs/Zp|=24, and the measurementprecision can be made sufficiently high.

Equivalent circuit simulation results obtained by using circuitconstants heretofore described are shown in FIGS. 12A-12C. FIG. 12Ashows a voltage ratio between V1 and V2 (indicated in FIG. 11) obtainedwhen the voltage measurement circuit is not connected, and shows thesame result as that of FIG. 6. V1/V2 ratio and V1/V3 ratio obtained whenthe voltage measurement circuit having the above-described circuitconstants is connected are shown in FIGS. 12B and 12C, respectively.Comparing the V1/V2 ratio shown in FIG. 12A with that shown in FIG. 12B,it is appreciated that an influence of the voltage measurement circuitis noticeable at 40 MHz or above, but the influence of the voltagemeasurement circuit is hardly noticeable at 10 MHz or below.

In the V1/V3 ratio shown in FIG. 12C, a drop in voltage ratio caused byresonance in the vicinity of 40 MHz is noticeable. Calculation of theresonant frequency in the voltage measurement circuit is conducted asdescribed below.

First, a composite impedance of L6, C6 and C7 shown in FIG. 11 will nowbe found. L6 corresponds to a coaxial cable having a length of 50 cm.Therefore, it follows that L6=0.135 μH. Since a composite capacitance ofC6 and C7 is 8 pF, the composite impedance of L6, C6 and C7 becomes −5ikΩ. Since this is a capacitive impedance, 8.005 pF is obtained byconverting the capacitive impedance into a capacitance.

A composite capacitance of this and C9 becomes 103 pF+8.005 pF=111.005pF. Since this composite capacitance and L5 (=0.135 μH) cause serialresonance, the resonant frequency (hereafter referred to asReso_Measure) becomes 41.113 MHz according to the Expression 1.

Because of a voltage variation caused by the resonance in the voltagemeasurement circuit, there must be a definite relation between thefrequency of the voltage to be measured and the resonant frequency.

In order to hold down the voltage measurement precision to ±5%, afrequency satisfying the relation V1/V3>0.95 is checked particularly inthe graph representing the V1/V3 ratio shown in FIG. 12C. As a result,the frequency is 8.9 MHz or below. Denoting the frequency of the voltageto be measured by fB, therefore, it follows thatReso_Measure/fB>41.113/8.9=4.6.

Denoting an inductance and a capacitance that determine the resonantfrequency of the voltage measurement circuit respectively by L and C,therefore, it is necessary to satisfy the following expression (6) onthe basis of the Expression (1).

LC<(9.2 τfB)⁻²  (6)

The coefficient 9.2 need not be this value necessarily. Since thiscoefficient depends on the voltage measurement precision, thiscoefficient should be determined with respect to a required measurementprecision by using simulation or actual measurement. For example, if themeasurement precision is set to +10% under the same condition as that inFIG. 12C, the frequency satisfying the relation V1/V3>0.90 becomes 12.6MHz or below, and the coefficient becomes 6.5 (=41.113/12.6*2).

Phase measurement will now be described. If a rectifier circuit using adiode D1 included in the voltage measurement circuit shown in FIGS. 8and 11 is replaced by a phase detection circuit, the voltage phase canbe measured. Block diagrams corresponding to FIGS. 8 and 11 are shown inFIGS. 13 and 14, respectively. Results obtained by simulating phasedifferences of V1/V2 and V1/V3 in FIG. 14 under the same conditions asFIGS. 12A-12C are shown in FIGS. 15A and 15B, respectively. The phasedifference between V1 and V2 exhibits complicated behavior. The phasedifference between V1 and V3 suddenly changes from 0° to 180° at aresonant frequency of 41 MHz. This is because the phase detectioncircuit is formed of only an inductance and a capacitance without usingresistances. If resistances are used, then unadvantageously the phasedifference exhibits a comparatively gently-sloping change. It isappreciated from this result that there are no problems as regards thephase measurement as long as the restriction represented by theExpression 6 is observed.

The circuit concerning the voltage measurement and phase measurementheretofore described can be applied to not only the electrode having awafer mounted thereon, but also all electrodes capacitance-coupled toplasma. This embodiment will now be described.

FIG. 16 is a longitudinal section diagram of an etching chamber used inthe present invention. FIG. 16 differs from FIG. 10 in that not only thehigh frequency power supply for plasma generation (the first highfrequency power supply) 108 (having a frequency of, for example, 200MHz) is connected to the antenna electrode 103 via the matcher 109 butalso an antenna bias power supply 113 which is a third high frequencypower supply is connected to the antenna electrode 103 via a matcher112. The antenna bias power supply 113 and the wafer bias power supply119 are connected to a phase controller 120. As a result, phases of thehigh frequencies output from the antenna bias power supply 113 and thewafer bias power supply 119 can be controlled. In this case, the antennabias power supply 113 and the wafer bias power supply 119 are made tohave the same frequency (for example, 4 MHz). In this system, adifference in phase (for example, 180°) between the antenna biasing highfrequency appearing on the antenna electrode 103 and the wafer biasinghigh frequency appearing on the wafer 116 is controlled, and a bias canbe applied to each of the antenna electrode 103 and the wafer 116effectively. For implementing such a system, the voltage and phase atthe electrostatic chuck electrode 124 are detected by pulling out thevoltage to the atmospheric pressure side by the use of the coaxial cable157 and providing a phase measurement circuit 155. In order to detectthe voltage and phase at the upper antenna electrode 103, the voltage atthe antenna electrode 103 is taken out to the atmospheric pressure sideby using a coaxial cable 159 and a phase measurement circuit 156 isprovided, in the same way as the lower electrode. A phase controller 120compares phases obtained from the two phase measurement circuits 155 and156, and determines a phase difference in high frequencies to be sent tothe antenna bias power supply 113 and the wafer bias power supply 119 soas to generate a predetermined phase difference.

In order to raise the reliability of the control, the matcher 109incorporates a filter 110 for cutting off the frequency of the antennabias power supply 113. In the same way, the matcher 112 incorporates afilter 121 for cutting off the frequency of the high frequency powersupply 108 for plasma generation. Outputs of the two matchers 109 and112 are combined by using a coaxial cable 158, and a resultant signal iscoupled to the coaxial line 111 which is the high frequency transmissionsystem for the antenna electrode.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A plasma processing apparatus including a vacuum vessel, a lower electrode provided in the vacuum vessel to place a sample thereon, a matcher connected to the lower electrode, and a power supply for supplying power to the lower electrode via the matcher, the plasma processing apparatus comprising: an electrostatic chuck electrode provided within the lower electrode to hold the sample; and a voltage measurement circuit provided within the lower electrode to measure a voltage at said electrostatic chuck electrode and output the measured voltage as a DC voltage.
 2. The plasma processing apparatus according to claim 1, wherein said voltage measurement circuit is installed within a vessel that intercepts at least a corrosive gas.
 3. The plasma processing apparatus according to claim 1, wherein said voltage measurement circuit can detect a phase signal.
 4. A plasma processing apparatus including a vacuum vessel, a lower electrode provided in the vacuum vessel so as to incorporate an electrostatic chuck electrode for holding a sample, a matcher connected to the lower electrode, and a power supply for supplying power to the lower electrode via the matcher, the plasma processing apparatus comprising: a voltage measurement circuit provided under atmospheric pressure to measure a voltage at the electrostatic chuck electrode and output the measured voltage as a DC voltage; and a coaxial line for connecting the electrostatic chuck electrode to said voltage measurement circuit.
 5. The plasma processing apparatus according to claim 4, wherein a composite impedance of said voltage measurement circuit and said coaxial line is greater than a load impedance between the electrostatic chuck electrode and plasma.
 6. The plasma processing apparatus according to claim 4, wherein said voltage measurement circuit can detect a phase signal.
 7. A plasma processing apparatus including a vacuum vessel, a lower electrode provided in the vacuum vessel so as to incorporate an electrostatic chuck electrode for holding a sample, an upper electrode provided in a position opposed to the lower electrode, a first matcher connected to the lower electrode, a first power supply for supplying power to the lower electrode via the first matcher, a second matcher connected to the upper electrode, and a second power supply for supplying power to the upper electrode via the second matcher, the plasma processing apparatus comprising: a first phase measurement circuit provided under atmospheric pressure to measure a phase of a voltage applied to the electrostatic chuck electrode; a first coaxial line for connecting the electrostatic chuck electrode to said first phase measurement circuit; a second phase measurement circuit provided under atmospheric pressure to measure a phase of a voltage applied to the upper electrode; a second coaxial line for connecting the upper electrode to said second phase measurement circuit; and a controller for controlling the first power supply and the second power supply based on output signals of said first phase measurement circuit and said second phase measurement circuit. 